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Abstract

We present a GPU accelerated toolbox for shaping the light propagation through multimode fibre using a spatial light modulator (SLM). The light is modulated before being coupled to the proximal end of the fibre in order to achieve arbitrary light patterns at the distal end of the fibre. First, the toolbox optimises the acquisition time of the transformation matrix of the fibre by synchronous operation of CCD and SLM. Second, it uses the acquired transformation matrix retained within the GPU memory to design, in real-time, the desired holographic mask for on-the-fly modulation of the output light field. We demonstrate the functionality of the toolbox by acquiring the transformation matrix at the maximum refresh rate of the SLM - 204Hz, and using it to display an on-demand oriented cube, at the distal end of the fibre. The user-controlled orientation of the cube and the corresponding holographic mask are obtained in 20ms intervals. Deleterious interference effects between the neighbouring points are eliminated by incorporating an acousto-optic deflector (AOD) into the system. We remark that the usage of the toolbox is not limited to multimode fibres and can be readily used to acquire transformation matrix and implement beam-shaping in any other linear optical system.

Figures (7)

Laser light from Verdi (5W; λ = 532nm) is magnified by a telescope in order to overfill the SLM plane. The complex superposition of two gratings and subsequent extraction of phase from the resulting complex matrix is applied on the phase-only SLM. The first grating generates a spot in (u, v) at the input facet of MMF (Thorlabs M14L01), the second grating focuses a spot on the input to SMF. The output fields from SMF and MMF are interfered on the CCD and the image is sent to the GPU memory. The phase of the spot in (u, v) is changed N times over a 2πn range and the whole process is repeated for all input modes (u, v) of the MMF. The resulting images are used to extract the phase and amplitude matrix Muv(x, y) of the output field in any point (x, y). Applying FFT to Muv(x, y) creates a hologram generating field at the fibre input side that results in a well defined spot in (x, y) at the fibre output.

The beam deflected by AOD impinges on the SLM at an angle given by the current frequency of AOD. The hologram on the SLM consists of the complex superposition of masks focusing light to different spots (x, y) at the fibre output. Each of the constituting masks has an added grating that cancels out one specific angle introduced by the AOD. Subsequently, only the input field corresponding to a mask with the right correction angle for a given AOD deflection will propagate through the fibre. In this way we can cycle between the spots at the output with a maximum refresh rate of the AOD without introducing any interference effects between the output spots.

Projected 3D cube, created using 120 output points, generated at the distal end of the MMF with a refresh rate of 50Hz. The contrast ratio of the cube lines to the fibre background light is 20 : 1 (
Media 1).

The FFT resolution influences the contrast and computational time of the generated pattern. The contrast is significantly worse only for 32 × 32 and 64 × 64 FFT and the computational time is significantly longer only for 512 × 512 FFT case. Therefore, the optimal trade-off between speed and contrast is for 128 × 128 or 256 × 256 FFT.